Dust filter bags that are made of a porous nonwoven fabric and a filter paper are known from European Patent 0 635 297 A1, and are processed into double-layer dust filter bags. A melt-blown fine fiber batt, which covers the inner side of the dust filter bag and reinforces the dust filter bag, can be used as the nonwoven fabric. However, the reinforcing effect presented by this approach is not satisfactory.
Further dust filter bags are known from European Patent 0 338 479 B1. The dust filter bag described therein comprises a filter-paper outer ply and an internally located nonwoven fabric. The nonwoven fabric is configured as a fine-fiber nonwoven fabric and is also arranged on the inflow side. The fine fibers of the fine-fiber nonwoven fabric can be applied in the thermoplastic state directly onto the filter paper and joined to it. The fine-fiber nonwoven fabric can be joined to a support element which is also made of nonwoven fabric. To produce the semifinished bag, a tube is formed from the laminate and is closed with a longitudinal seam. Pieces of finite length are then cut off from the endless tube on a bottom-seam drum. An air pulse is used to open the tube ends on one side in order to ensure the formation of tabs which are turned over and adhesively bonded to one another. However, because the two internally located nonwoven fiber plies can easily weld to one another during the cutting process, they can no longer reliably be opened with the air pulse.
German Patent 196 06 718 A1 furthermore discloses a multiple-ply filter pouch that has a first inner ply made of a fine-fiber nonwoven fabric, a second outer ply made of a plastic matted-fiber material, and a third ply, arranged before the first ply in the flow direction, made of a paper material. This creates the filtration effectiveness of the ply made of paper material, since with an external arrangement, the actual filtering effect occurs first.
A further disadvantage of the known dust filter pouches is the fact that when water or other fluid is drawn in along with the air being cleaned, the filter paper ply softens and its strength is impaired, creating the risk of tearing and contamination of the environment during vacuum cleaning or when the dust filter bag is removed. The filtering effect in terms of ultrafine particles is less than satisfactory.